When one thinks about a typical Autumn in the American Northeast, changing leaves and vibrant pumpkins come to mind. Or perhaps, the ability to comfortably wear a sweater. Few consider the forces hard at work to create the circumstances for such fall festivities.
Why do we have Fall weather?
Winds that travel from the eastern to western ends of the Pacific, known as trade winds, bring warm South American surface waters to the coasts of eastern Asia. This yearly occurrence takes place around the beginning of the fall season.
While the warmer waters are being escorted away from the western coast of South America, colder waters from the southern Pacific quickly move into the recently vacated region. This process is known as upwelling.
So, at this point in the cycle, warmer waters surround east Asian coasts, and cooler waters envelop western beaches in South America. Under these conditions, warmer waters in eastern Asia cause warm air containing high levels of moisture to rise, resulting in variable weather and frequent precipitation in this area. Inversely, the rising of cooler, drier air leads to chilly conditions that lack precipitation.
This cycle continues until El Niño weather patterns develop.
What is El Niño?
Under proper circumstances in the Pacific, El Niño conditions may develop. When abnormal weather patterns occur in the Pacific Ocean, such as systems of above-normal strength in the tropical region, or a decrease in equatorial weather systems, the trade winds crucial to providing stable weather across the globe become weaker, and in some cases, they even blow in the opposite direction.
These weakened trade winds are less capable of pushing warmer Pacific waters to the western end of the ocean. As a result, there is not an absence of warm water in the East Pacific that must be replaced with cooler, southern waters. Thus, the usual cycle of upwelling is suspended; warm waters stay surrounding western South America and cold waters stay in eastern Asia.
The impact of this situation is felt around the globe. During El Niño, the Pacific jet stream, an air current that travels across the United States horizontally, is shifted further South and East. This translates to extremely wet conditions in the southern United States and drought-like conditions in the North. The effects of El Niño are felt globally, as energy is released into the atmosphere, causing up global temperatures to surge.
What is La Niña?
Sometimes, years following El Niño conditions reverse into patterns characteristic of its near cousin: El Niño. During La Niña conditions, Pacific trade winds strengthen tremendously, pushing the Pacific’s warmest waters to its western edge. As a result of the extreme movement of the warm waters, upwelling is increased since there is a larger amount of warm water to be replaced by cooler, southern waters.
La Niña weakens the Pacific jet stream and drives it into the northern United States. As the circumstances of La Niña conditions are the inverse of El Niño’s, the impacts are as well. The northern United States experiences drier and milder winters, while the South is bombarded with unusually cold weather and high instances of precipitation. Additionally, the Atlantic Ocean’s hurricane season is typically longer and more active.
Global temperatures plunge during La Niña, and wildfires and droughts become more frequent occurrences nearly everywhere.
Why does La Niña matter?
El Niño and La Niña increase variability and the strength of weather patterns across the globe. The significance of the two weather circumstances is clear when trying to prepare for and prevent extreme weather events.
The National Oceanic and Atmospheric Administration’s Climate Prediction Center has recently confirmed that La Niña will be in effect from December to February of next year. Climate patterns caused by this condition will make for a severe Winter. It is imperative that all members of the MLHS community are aware and prepared!
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